Deletion of the Glutaredoxin-2 Gene Protects Mice from Diet-Induced Weight Gain, Which Correlates with Increased Mitochondrial Respiration and Proton Leaks in Skeletal Muscle

The emerging importance of the α-keto acid dehydrogenase complexes in serving as intracellular and intercellular signaling platforms for the regulation of metabolism

The α-keto acid dehydrogenase complex (KDHc) class of mitochondrial enzymes is composed of four members: pyruvate dehydrogenase (PDHc), α-ketoglutarate dehydrogenase (KGDHc), branched-chain keto acid dehydrogenase (BCKDHc), and 2-oxoadipate dehydrogenase (OADHc). These enzyme complexes occupy critical metabolic intersections that connect monosaccharide, amino acid, and fatty acid metabolism to Krebs cycle flux and oxidative phosphorylation (OxPhos). This feature also imbues KDHc enzymes with the heightened capacity to serve as platforms for propagation of intracellular and intercellular signaling. KDHc enzymes serve as a source and sink for mitochondrial hydrogen peroxide (mtH2O2), a vital second messenger used to trigger oxidative eustress pathways. Notably, deactivation of KDHc enzymes through reversible oxidation by mtH2O2 and other electrophiles modulates the availability of several Krebs cycle intermediates and related metabolites which serve as powerful intracellular and intercellular messengers. The KDHc enzymes also play important roles in the modulation of mitochondrial metabolism and epigenetic programming in the nucleus through the provision of various acyl-CoAs, which are used to acylate proteinaceous lysine residues. Intriguingly, nucleosomal control by acylation is also achieved through PDHc and KGDHc localization to the nuclear lumen. In this review, I discuss emerging concepts in the signaling roles fulfilled by the KDHc complexes. I highlight their vital function in serving as mitochondrial redox sensors and how this function can be used by cells to regulate the availability of critical metabolites required in cell signaling. Coupled with this, I describe in detail how defects in KDHc function can cause disease states through the disruption of cell redox homeodynamics and the deregulation of metabolic signaling. Finally, I propose that the intracellular and intercellular signaling functions of the KDHc enzymes are controlled through the reversible redox modification of the vicinal lipoic acid thiols in the E2 subunit of the complexes.

The impact of glutaredoxin 1 and glutaredoxin 2 double knockout on lens epithelial cell function

Glutaredoxins (Grx1 and Grx2) are thiol-repair antioxidant enzymes that play vital roles in cellular redox homeostasis and various cellular processes. This study aims to evaluate the functions of the glutaredoxin (Grx) system, including glutaredoxin 1 (Grx1) and glutaredoxin 2 (Grx2), using Grx1/Grx2 double knockout (DKO) mice as a model. We isolated primary lens epithelial cells (LECs) from wild-type (WT) and DKO mice for a series of in vitro analyses. Our results revealed that Grx1/Grx2 DKO LECs exhibited slower growth rates, reduced proliferation, and aberrant cell cycle distribution compared to WT cells. Elevated levels of β-galactosidase activity were observed in DKO cells, along with a lack of caspase 3 activation, suggesting that these cells may be undergoing senescence. Additionally, DKO LECs displayed compromised mitochondrial function, characterized by decreased ATP production, reduced expression levels of oxidative phosphorylation (OXPHOS) complexes III and IV, and increased proton leak. A compensatory metabolic shift towards glycolysis was observed in DKO cells, indicating an adaptive response to Grx1/Grx2 deficiency. Furthermore, loss of Grx1/Grx2 affected cellular structure, leading to increased polymerized tubulin, stress fiber formation, and vimentin expression in LECs. In conclusion, our study demonstrates that Grx1/Grx2 double deletion in LECs results in impaired cell proliferation, aberrant cell cycle progression, disrupted apoptosis, compromised mitochondrial function, and altered cytoskeletal organization. These findings underscore the importance of Grx1 and Grx2 in maintaining cellular redox homeostasis and the consequences of their deficiency on cellular structure and function. Further research is needed to elucidate the precise molecular mechanisms underlying these observations and to investigate potential therapeutic strategies targeting Grx1 and Grx2 for various physiological processes and oxidative-stress related diseases such as cataract.

S-nitroso-glutathione (GSNO) inhibits hydrogen peroxide production by alpha-ketoglutarate dehydrogenase: An investigation into sex and diet effects

Pyruvate dehydrogenase (PDH) and α-ketoglutarate dehydrogenase (KGDH) are vital sources of hydrogen peroxide (H₂O₂) and key sites for redox regulation. Here, we report KGDH is more sensitive to inhibition by S-nitroso-glutathione (GSNO) when compared to PDH and deactivation of both enzymes by nitro modification is affected by sex and diet. Liver mitochondria from male C57BL/6N mice displayed a robust inhibition of H₂O₂ production after exposure to 500-2000 μM GSNO. H₂O₂ genesis by PDH was not significantly affected by GSNO. Purified KGDH of porcine heart origin displayed a ∼82% decrease in H₂O₂ generating activity at 500 μM GSNO, which was mirrored by a decrease in NADH production. By contrast, H₂O₂- and NADH-producing activity of purified PDH was only minimally affected by an incubation in 500 μM GSNO. Incubations in GSNO had no significant effect on the H₂O₂-generating activity of KGDH and PDH in female liver mitochondria when compared to samples collected from males, which was attributed to higher GSNO reductase (GSNOR) activity. High fat feeding augmented the GSNO-mediated inhibition of KGDH in liver mitochondria from male mice. Exposure of male mice to a HFD also resulted in a significant decrease in the GSNO-mediated inhibition of H₂O₂ genesis by PDH, an effect not observed in mice fed a control-matched diet (CD). Female mice displayed higher resistance to the GSNO-induced inhibition of H₂O₂ production, regardless of being fed a CD or HFD. However, exposure to a HFD did result in a small but significant decrease in H₂O₂ production by KGDH and PDH when female liver mitochondria were treated with GSNO. Although, the effect was less when compared to their male counterparts. Collectively, we show for the first time GSNO deactivates H₂O₂ production by α-keto acid dehydrogenases and we demonstrate that sex and diet are determinants for the nitro-inhibition of both KGDH and PDH.

Protein S-glutathionylation and sex dimorphic effects on hydrogen peroxide production by dihydroorotate dehydrogenase in liver mitochondria

Dihydroorotate dehydrogenase (DHODH) oxidizes dihydroorotate to orotate for pyrimidine biosynthesis, donating electrons to the ubiquinone (UQ) pool of mitochondria. DHODH has a measurable rate for hydrogen peroxide (H₂O₂) production and thus contributes to cellular changes in redox tone. Protein S-glutathionylation serves as a negative feedback loop for the inhibition of H₂O₂ by several α-keto acid dehydrogenases and respiratory complexes in mitochondria, as well as ROS sources in liver cytoplasm. Here, we report this redox signaling mechanism also inhibits H₂O₂ production by DHODH in liver mitochondria isolated from male and female C57BL6N mice. We discovered that low amounts of the glutathionylation catalyst, disulfiram (50-500 nM), almost abolished H₂O₂ production by DHODH in mitochondria from male mice. Similar results were collected with diamide, however, higher doses (1000-5000 μM) were required to elicit this effect. Disulfiram and diamide also significantly suppressed H₂O₂ production by DHODH in female liver mitochondria. However, liver mitochondria from female mice were more resistant to disulfiram or diamide-mediated inhibition of H₂O₂ genesis when compared to samples from males. Analysis of the impact of disulfiram and diamide on DHODH activity revealed that both compounds inhibited the dehydrogenase directly, however the effect was less in female mice. Additionally, disulfiram and diamide impeded the use of dihydroorotate fueled oxidative phosphorylation in mitochondria from males and females, although samples collected from female rodents displayed more resistance to this inhibition. Taken together, our findings demonstrate H₂O₂ production by DHODH can be inhibited by glutathionylation and sex can impact this redox modification.

Regulation of Mitochondrial Hydrogen Peroxide Availability by Protein S-glutathionylation

BACKGROUND

It has been four decades since protein S-glutathionylation was proposed to serve as a regulator of cell metabolism. Since then, this redox-sensitive covalent modification has been identified as a cell-wide signaling platform required for embryonic development and regulation of many physiological functions.

SCOPE OF THE REVIEW

Mitochondria use hydrogen peroxide (H₂O₂) as a second messenger, but its availability must be controlled to prevent oxidative distress and promote changes in cell behavior in response to stimuli. Experimental data favor the function of protein S-glutathionylation as a feedback loop for the inhibition of mitochondrial H₂O₂ production.

MAJOR CONCLUSIONS

The glutathione pool redox state is linked to the availability of H₂O₂, making glutathionylation an ideal mechanism for preventing oxidative distress whilst playing a part in desensitizing mitochondrial redox signals.

GENERAL SIGNIFICANCE

The biological significance of glutathionylation is rooted in redox status communication. The present review critically evaluates the experimental evidence supporting its role in negating mitochondrial H₂O₂ production for cell signaling and prevention of electrophilic stress.

Conditions Conducive to the Glutathionylation of Complex I Subunit NDUFS1 Augment ROS Production following the Oxidation of Ubiquinone Linked Substrates, Glycerol-3-Phosphate and Proline

Mitochondrial complex I can produce large quantities of reactive oxygen species (ROS) by reverse electron transfer (RET) from the ubiquinone (UQ) pool. Glutathionylation of complex I does induce increased mitochondrial superoxide/hydrogen peroxide (O2●-/H2O2) production, but the source of this ROS has not been identified. Here, we interrogated the glutathionylation of complex I subunit NDUFS1 and examined if its modification can result in increased ROS production during RET from the UQ pool. We also assessed glycerol-3-phosphate dehydrogenase (GPD) and proline dehydrogenase (PRODH) glutathionylation since both flavoproteins have measurable rates for ROS production as well. Induction of glutathionylation with disulfiram induced a significant increase in O2●-/H2O2 production during glycerol-3-phosphate (G3P) and proline (Pro) oxidation. Treatment of mitochondria with inhibitors for complex I (rotenone and S1QEL), complex III (myxothiazol and S3QEL), glycerol-3-phosphate dehydrogenase (iGP), and proline dehydrogenase (TFA) confirmed that the sites for this increase were complexes I and III, respectively. Treatment of liver mitochondria with disulfiram (50-1000 nM) did not induce GPD or PRODH glutathionylation, nor did it affect their activities, even though disulfiram dose-dependently increased the total number of protein glutathione mixed disulfides (PSSG). Immunocapture of complex I showed disulfiram incubations resulted in the modification of NDUFS1 subunit in complex I. Glutathionylation could be reversed by reducing agents, restoring the deglutathionylated state of NDUFS1 and the activity of the complex. Reduction of glutathionyl moieties in complex I also significantly decreased ROS production by RET from GPD and PRODH. Overall, these findings demonstrate that the modification of NDUFS1 can result in increased ROS production during RET from the UQ pool, which has implications for understanding the relationship between mitochondrial glutathionylation reactions and induction of oxidative distress in several pathologies.

Mitochondrial Glrx2 Knockout Augments Acetaminophen-Induced Hepatotoxicity in Mice

Acetaminophen (APAP) is one of the most widely used drugs with antipyretic and analgesic effects, and thus hepatotoxicity from the overdose of APAP becomes one of the most common forms of drug-induced liver injury. The reaction towards thiol molecules, such as GSH by APAP metabolite, N-acetyl-p-benzo-quinonimine (NAPQI), is the main cause of APAP-induced hepatotoxicity. However, the role of many other thiol-related regulators in toxicity caused by APAP is still unclear. Here we have found that knockout of the Glrx2 gene, which encodes mitochondrial glutaredoxin2 (Grx2), sensitized mice to APAP-caused hepatotoxicity. Glrx2 deletion hindered Nrf2-mediated compensatory recovery of thiol-dependent redox systems after acetaminophen challenge, resulting in a more oxidized cellular state with a further decrease in GSH level, thioredoxin reductase activity, and GSH/GSSG ratio. The weakened feedback regulation capacity of the liver led to higher levels of protein glutathionylation and thioredoxin (both Trx1 and Trx2) oxidation in Glrx2^−/− mice. Following the cellular environment oxidation, nuclear translocation of apoptosis-inducing factor (AIF) was elevated in the liver of Glrx2^−/− mice. Taken together, these results demonstrated that mitochondrial Grx2 deficiency deteriorated APAP-induced hepatotoxicity by interrupting thiol-redox compensatory response, enhancing the AIF pathway-mediated oxidative damage.

The Uncoupling Proteins: A Systematic Review on the Mechanism Used in the Prevention of Oxidative Stress

Mitochondrial uncoupling proteins (UCP) 1-3 fulfill many physiological functions, ranging from non-shivering thermogenesis (UCP1) to glucose-stimulated insulin release (GSIS) and satiety signaling (UCP2) and muscle fuel metabolism (UCP3). Several studies have suggested that UCPs mediate these functions by facilitating proton return to the matrix. This would decrease protonic backpressure on the respiratory chain, lowering the production of hydrogen peroxide (H₂O₂), a second messenger. However, controlling mitochondrial H₂O₂ production to prevent oxidative stress by activating these leaks through these proteins is still enthusiastically debated. This is due to compelling evidence that UCP2/3 fulfill other function(s) and the inability to reproduce findings that UCP1-3 use inducible leaks to control reactive oxygen species (ROS) production. Further, other studies have found that UCP2/3 may serve as Ca²⁺. Therefore, we performed a systematic review aiming to summarize the results collected on the topic. A literature search using a list of curated keywords in Pubmed, BIOSIS Citation Index and Scopus was conducted. Potentially relevant references were screened, duplicate references eliminated, and then literature titles and abstracts were evaluated using Rayyan software. A total of 1101 eligible studies were identified for the review. From this total, 416 studies were evaluated based on our inclusion criteria. In general, most studies identified a role for UCPs in preventing oxidative stress, and in some cases, this may be related to the induction of leaks and lowering protonic backpressure on the respiratory chain. However, some studies also generated evidence that UCP2/3 may mitigate oxidative stress by transporting Ca²⁺ into the matrix, exporting lipid hydroperoxides, or by transporting C-4 metabolites. Additionally, some showed that activating UCP1 or 3 can increase mitochondrial ROS production, even though there is still augmented protection from oxidative stress. Conclusion: Overall, most available studies demonstrate that UCPs, particularly UCP2/3, prevent oxidative stress. However, the mechanism utilized to do so remains elusive and raises the question that UCP2/3 should be renamed, since they may still not be true “uncoupling proteins”.

The glutathionylation agent disulfiram augments superoxide/hydrogen peroxide production when liver mitochondria are oxidizing ubiquinone pool-linked and branched chain amino acid substrates

Our group has previously observed that protein S-glutathionylation serves as an integral feedback inhibitor for the production of superoxide (O₂^●-)/hydrogen peroxide (H₂O₂) by α-ketoglutarate dehydrogenase (KGDH), pyruvate dehydrogenase (PDH), and complex I in muscle and liver mitochondria, respectively. In the present study, we hypothesized that glutathionylation would fulfill a similar role for the O₂^●-/H₂O₂ sources sn-glycerol-3-phosphate dehydrogenase (G3PDH), proline dehydrogenase (PRODH), and branched chain keto acid dehydrogenase (BCKDH). Surprisingly, we found that inducing glutathionylation with disulfiram increased the production of O₂^●-/H₂O₂ by mitochondria oxidizing glycerol-3-phosphate (G3P), proline (Pro), or α-keto-β-methylvaleric acid (KMV). Treatment of mitochondria oxidizing G3P or Pro with rotenone or myxothiazol increased the rate of ROS production after incubating in 1000 nM disulfiram. Incubating mitochondria treated with disulfiram in both rotenone and myxothiazol prevented this increase in O₂^●-/H₂O₂ production. In addition, when adminstered together, ROS production decreased below control levels. Disulfiram-treated mitochondria displayed higher rates of ROS production when oxidizing succinate, which was inhibited by rotenone, myxothiazol, and malonate, respectively. Disulfiram also increased ROS production by mitocondria oxidizing KMV. Treatment of mitochondria oxidizing KMV with disulfiram and rotenone or myxothiazol did not alter the rate O₂^●-/H₂O₂ production further when compared to mitochondria treated with disulfiram only. Analysis of BCKDH activity following disulfiram treatment revealed that glutathionylation does not inhibit the enzyme complex, indicating this α-keto acid dehydrogenase is not a target for glutathione modification. However, treatment of mitochondria with rotenone and myxothiazol without disulfiram also augmented ROS production. Overall, we were able to demonstrate for the first time that glutathionylation augments ROS production by the respiratory chain during forward electron transfer (FET) and reverse electron transfer (RET) from the UQ pool. Additionally, we were able to show that BCKDH is not a target for glutathione modification and that glutathionylation can also increase ROS production in mitochondria oxidizing branched chain amino acids following the modification of enzymes upstream of BCKDH.

Thiol-based redox-active proteins as cardioprotective therapeutic agents in cardiovascular diseases

Thiol-based redox compounds, namely thioredoxins (Trxs), glutaredoxins (Grxs) and peroxiredoxins (Prxs), stand as a pivotal group of proteins involved in antioxidant processes and redox signaling. Glutaredoxins (Grxs) are considered as one of the major families of proteins involved in redox regulation by removal of S-glutathionylation and thereby reactivation of other enzymes with thiol-dependent activity. Grxs are also coupled to Trxs and Prxs recycling and thereby indirectly contribute to reactive oxygen species (ROS) detoxification. Peroxiredoxins (Prxs) are a ubiquitous family of peroxidases, which play an essential role in the detoxification of hydrogen peroxide, aliphatic and aromatic hydroperoxides, and peroxynitrite. The Trxs, Grxs and Prxs systems, which reversibly induce thiol modifications, regulate redox signaling involved in various biological events in the cardiovascular system. This review focuses on the current knowledge of the role of Trxs, Grxs and Prxs on cardiovascular pathologies and especially in cardiac hypertrophy, ischemia/reperfusion (I/R) injury and heart failure as well as in the presence of cardiovascular risk factors, such as hypertension, hyperlipidemia, hyperglycemia and metabolic syndrome. Further studies on the roles of thiol-dependent redox systems in the cardiovascular system will support the development of novel protective and therapeutic strategies against cardiovascular diseases.

Undaria pinnatifida improves obesity-related outcomes in association with gut microbiota and metabolomics modulation in high-fat diet-fed mice

Dietary fiber has beneficial effects on obesity-related diseases and gut microbiota, contributing a key role in the interaction between dietary metabolism and host metabolism. Our objective was to investigate the cause of the improvement in multiple types of physiological states with seaweed Undaria pinnatifida treatment on high-fat diet-fed mice and to evaluate whether its consequent anti-adiposity and anti-hyperlipidemic effects are associated with gut microbiota and its metabolomics regulation. U. pinnatifida administration in our experiment was shown to significantly decrease high-fat diet-induced body weight gain, as well as epididymal and abdominal adiposity. U. pinnatifida intake also significantly reduced liver weight and serum triacylglycerol accumulation. We also found that improving effects of U. pinnatifida on high-fat diet-induced metabolic dysfunctions were associated with significant increase in specific bacteria, such as Bacteroides acidifaciens and Bacteroides ovatus, as well as metabolites, including short-chain fatty acids and tricarboxylic acid cycle intermediates. Our result provides a cheap dietary strategy to host metabolism improvement and obesity management. KEY POINTS: • U. pinnatifida improved adipose accumulation and lipid metabolism. • B. acidifaciens and B. ovatus contributed to the beneficial effects of U. pinnatifida. • SCFAs and TCA cycle intermediates were critical to the metabolic outcomes. • Our study provides a cheap dietary strategy for obesity management.

An investigation into the impact of deleting one copy of the glutaredoxin-2 gene on diet-induced weight gain and the bioenergetics of muscle mitochondria in female mice fed a high fat diet

Our group recently documented that male mice containing a deletion for one copy of the glutaredoxin-2 (Grx2) gene were completely protected from developing diet-induced obesity (DIO). Objectives: Here, we conducted a similar investigation but with female littermates. Results: In comparison to our recent publication using male mice, exposure of WT and GRX2+/- female mice to a HFD from 3-to-10 weeks of age did not induce any changes in body mass, circulating blood glucose, food intake, hepatic glycogen levels, or abdominal fat pad mass. Examination of the bioenergetics of muscle mitochondria revealed no changes in the rate of superoxide ( O 2 ∙ - )/hydrogen peroxide (H₂O₂) or O₂ consumption under different states of respiration or alterations in lipid peroxidation adduct levels regardless of mouse strain or diet. Additionally, we measured the bioenergetics of mitochondria isolated from liver tissue and found that partial loss of GRX2 augmented respiration but did not alter ROS production. Discussion: Overall, our findings demonstrate there are sex differences in the protection of female GRX2+/- mice from DIO, fat accretion, intrahepatic lipid accumulation, and the bioenergetics of mitochondria from muscle and liver tissue.

Glutaredoxins with iron-sulphur clusters in eukaryotes - Structure, function and impact on disease

Among the thioredoxin superfamily of proteins, the observation that numerous glutaredoxins bind iron-sulphur (Fe/S) clusters is one of the more recent and major developments concerning their functional properties. Glutaredoxins are present in most organisms. All members of the class II subfamily (including most monothiol glutaredoxins), but also some members of the class I (mostly dithiol glutaredoxins) and class III (land plant-specific monothiol or dithiol glutaredoxins) are Fe/S proteins. In glutaredoxins characterised so far, the [2Fe2S] cluster is coordinated by two active-site cysteine residues and two molecules of non-covalently bound glutathione in homo-dimeric complexes bridged by the cluster. In contrast to dithiol glutaredoxins, monothiol glutaredoxins possess no or very little oxidoreductase activity, but have emerged as important players in cellular iron metabolism. In this review we summarise the recent developments of the most prominent Fe/S glutaredoxins in eukaryotes, the mitochondrial single domain monothiol glutaredoxin 5, the chloroplastic single domain monothiol glutaredoxin S14 and S16, the nuclear/cytosolic multi-domain monothiol glutaredoxin 3, and the mitochondrial/cytosolic dithiol glutaredoxin 2.

Role of Glutaredoxin-1 and Glutathionylation in Cardiovascular Diseases

Cardiovascular diseases are the leading cause of death worldwide, and as rates continue to increase, discovering mechanisms and therapeutic targets become increasingly important. An underlying cause of most cardiovascular diseases is believed to be excess reactive oxygen or nitrogen species. Glutathione, the most abundant cellular antioxidant, plays an important role in the body’s reaction to oxidative stress by forming reversible disulfide bridges with a variety of proteins, termed glutathionylation (GSylation). GSylation can alter the activity, function, and structure of proteins, making it a major regulator of cellular processes. Glutathione-protein mixed disulfide bonds are regulated by glutaredoxins (Glrxs), thioltransferase members of the thioredoxin family. Glrxs reduce GSylated proteins and make them available for another redox signaling cycle. Glrxs and GSylation play an important role in cardiovascular diseases, such as myocardial ischemia and reperfusion, cardiac hypertrophy, peripheral arterial disease, and atherosclerosis. This review primarily concerns the role of GSylation and Glrxs, particularly glutaredoxin-1 (Glrx), in cardiovascular diseases and the potential of Glrx as therapeutic agents.

Role of GSH and Iron-Sulfur Glutaredoxins in Iron Metabolism-Review

Glutathione (GSH) was initially identified and characterized for its redox properties and later for its contributions to detoxification reactions. Over the past decade, however, the essential contributions of glutathione to cellular iron metabolism have come more and more into focus. GSH is indispensable in mitochondrial iron-sulfur (FeS) cluster biosynthesis, primarily by co-ligating FeS clusters as a cofactor of the CGFS-type (class II) glutaredoxins (Grxs). GSH is required for the export of the yet to be defined FeS precursor from the mitochondria to the cytosol. In the cytosol, it is an essential cofactor, again of the multi-domain CGFS-type Grxs, master players in cellular iron and FeS trafficking. In this review, we summarize the recent advances and progress in this field. The most urgent open questions are discussed, such as the role of GSH in the export of FeS precursors from mitochondria, the physiological roles of the CGFS-type Grx interactions with BolA-like proteins and the cluster transfer between Grxs and recipient proteins.

Protein S-glutathionylation reactions as a global inhibitor of cell metabolism for the desensitization of hydrogen peroxide signals

The pathogenesis of many human diseases has been attributed to the over production of reactive oxygen species (ROS), particularly superoxide (O₂^●-) and hydrogen peroxide (H₂O₂), by-products of metabolism that are generated by the premature reaction of electrons with molecular oxygen (O₂) before they reach complex IV of the respiratory chain. To date, there are 32 known ROS generators in mammalian cells, 16 of which reside inside mitochondria. Importantly, although these ROS are deleterious at high levels, controlled and temporary bursts in H₂O₂ production is beneficial to mammalian cells. Mammalian cells use sophisticated systems to take advantage of the second messaging properties of H₂O₂. This includes controlling its availability using antioxidant systems and negative feedback loops that inhibit the genesis of ROS at sites of production. At its core, ROS production depends on fuel metabolism. Therefore, desensitizing H₂O₂ signals would also require the temporary inhibition of fuel combustion and fluxes through metabolic pathways that promote ROS production. Additionally, this would also demand the diversion of fuels and nutrients into pathways that generate NADPH and other molecules required to maintain cellular redox buffering capacity. Therefore, fuel selection and metabolic flux plays an integral role in dictating the strength and duration of cellular redox signals. In the present review I provide an updated view on the function of protein S-glutathionylation, a ubiquitous redox sensitive modification involving the formation of a disulfide between the small molecular antioxidant glutathione and a cysteine residue, in the regulation of cellular metabolism on a global scale. To date, these concepts have mostly been reviewed at the level of mitochondrial bioenergetics in the contexts of health and disease. Careful examination of the literature revealed that glutathionylation is a temporary inhibitor of most metabolic pathways including glycolysis, the Krebs cycle, oxidative phosphorylation, amino acid metabolism, and fatty acid combustion, resulting in the diversion of fuels towards NADPH-producing pathways and the inhibition of ROS production. Armed with this information, I propose that protein S-glutathionylation reactions desensitize H₂O₂ signals emanating from catabolic pathways using a three-pronged regulatory mechanism; 1) inhibition of metabolic flux through pathways that promote ROS production, 2) diversion of metabolites towards pathways that support antioxidant defenses, and 3) direct inhibition of ROS-generating enzymes.

C57BL/6J mice upregulate catalase to maintain the hydrogen peroxide buffering capacity of liver mitochondria

Here, we demonstrate that the upregulation of catalase is required to compensate for the loss of nicotinamide nucleotide transhydrogenase (NNT) to maintain hydrogen peroxide (H₂O₂) steady-state levels in C57BL/6J liver mitochondria. Our investigations using the closely related mouse strains C57BL/6NJ (6NJ; +NNT) and C57BL/6J (6J; -NNT) revealed that NNT is required for the provision of NADPH and that the upregulation of isocitrate dehydrogenase-2 (IDH2) activity is not enough to compensate for the absence of NNT, which is consistent with previous observations. Intriguingly, despite the absence of NNT, 6J mitochondria had rates of H₂O₂ production (58.56 ± 3.79 pmol mg^-1 min^-1) that were similar to samples collected from 6NJ mice (72.75 ± 14.26 pmol mg^-1 min^-1) when pyruvate served as the substrate. However, 6NJ mitochondria energized with succinate produced significantly less H₂O₂ (59.95 ± 2.13 pmol mg^-1 min^-1) when compared to samples from 6J mice (116.39 ± 20.74 pmol mg^-1 min^-1), an effect that was attributed to the presence of NNT. Further investigations into the H₂O₂ eliminating capacities of these mitochondria led to the novel observation that 6J mitochondria compensate for the loss of NNT by upregulating catalase. Indeed, 6NJ and 6J mitochondria energized with pyruvate or succinate displayed similar rates for H₂O₂ elimination, quenching ~84% and ~86% of the H₂O₂, respectively, in the surrounding medium within 30 s. However, inclusion of palmitoyl-CoA, an NNT inhibitor, significantly limited H₂O₂ degradation by 6NJ mitochondria only (~55% of H₂O₂ eliminated in 30 s). Liver mitochondria from 6J mice treated with palmitoyl-CoA still cleared ~80% of the H₂O₂ from the surrounding environment. Inhibition of catalase with triazole compromised the capacity of 6J mitochondria to maintain H₂O₂ steady-state levels. By contrast, disabling NADPH-dependent antioxidant systems had a limited effect on the H₂O₂ clearing capacity of 6J mitochondria. Liver mitochondria collected from 6NJ mice, on the other hand, were more reliant on the GSH and TRX systems to clear exogenously added H₂O₂. However, catalase still played an integral in eliminating H₂O₂ in 6NJ liver mitochondria. Immunoblot analyses demonstrated that catalase protein levels were ~7.7-fold higher in 6J mitochondria. Collectively, our findings demonstrate for the first time that 6J liver mitochondria compensate for the loss of NNT by increasing catalase levels for the maintenance of H₂O₂ steady-state levels. In general, our observations reveal that catalase is an integral arm of the antioxidant response in liver mitochondria.

Lactate dehydrogenase supports lactate oxidation in mitochondria isolated from different mouse tissues

Research over the past seventy years has established that mitochondrial-l-lactate dehydrogenase (m-L-LDH) is vital for mitochondrial bioenergetics. However, in recent report, Fulghum et al. concluded that lactate is a poor fuel for mitochondrial respiration [1]. In the present study, we have followed up on these findings and conducted an independent investigation to determine if lactate can support mitochondrial bioenergetics. We demonstrate herein that lactate can fuel the bioenergetics of heart, muscle, and liver mitochondria. Lactate was just as effective as pyruvate at stimulating mitochondrial coupling efficiency. Inclusion of LDH (sodium oxamate or GSK 2837808A) and pyruvate dehydrogenase (PDH; CPI-613) inhibitors abolished respiration in mitochondria energized with lactate. Lactate also fueled mitochondrial ROS generation and was just as effective as pyruvate at stimulating H₂O₂ production. Additionally, lactate-induced ROS production was inhibited by both LDH and PDH inhibitors. Enzyme activity measurements conducted on permeabilized mitochondria revealed that LDH is localized in mitochondria. In aggregate, we can conclude that mitochondrial LDH fuels bioenergetics in several tissues by oxidizing lactate.

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Lactate can fuel mitochondrial respiration.

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Lactate serves as a substrate for H₂O₂ production.

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Mitochondria contain LDH.